Post actualizado el día octubre 25, 2026 by DeiviSanzPlay
Let’s cut to the chase once and for all: there are 8 planets in the Solar System, not 9. Pluto was downgraded to a dwarf planet in 2006 by the International Astronomical Union. But since the original title talked about 9, here I explain why people still say that and what’s really out there. What comes next isn’t your typical school article. It’s what I’ve learned from reading NASA and ESA missions and the occasional paper that makes you rethink everything.
Mercury: the planet that barely has an atmosphere and either fries you or freezes you
Mercury is the planet closest to the Sun and, contrary to what many believe, it’s not the hottest. That title goes to Venus. Mercury has an atmosphere so thin it’s practically a vacuum, with loose atoms of sodium, oxygen, and helium stripped away by the solar wind. That makes the temperature swing from 430 °C during the day to -180 °C at night. A brutal contrast.
Its surface is covered in craters and plains of solidified lava. NASA’s MESSENGER mission confirmed in 2012 that there is water ice at the poles, in areas that never receive sunlight. Yes, ice on the planet closest to the Sun. Sounds like a joke, but it’s real.
Mercury completes an orbit in 88 Earth days. Hence its name, the swift messenger of the Roman gods. But don’t trust the mythology: its rotation is 59 days, so a solar day on Mercury lasts 176 Earth days. Chaos.
Venus: the hell we mistake for a star
Venus is the hottest planet in the Solar System, with a surface temperature of about 465 °C. Enough to melt lead. Its atmosphere is 90 times denser than Earth’s and is composed of almost 96% carbon dioxide. That generates a runaway greenhouse effect that traps all the heat.
That’s why it shines so brightly in the sky. We call it the «Morning Star» or «Evening Star,» but it’s a rocky planet wrapped in clouds of sulfuric acid. The Soviet Venera probes managed to land on its surface in the 1970s and 1980s, though they lasted barely an hour before being crushed and melted.
In 2020, a study led by Jane Greaves (Cardiff University) detected phosphine in Venus’s clouds. There was speculation about microbial life, but later analyses called it into question. Even so, NASA has two approved missions for the 2030s: VERITAS and DAVINCI. ESA will also launch EnVision. Venus is back in fashion.
Earth: the only place where we know there’s life (for now)
Earth is a rare case. It has liquid water on its surface, an atmosphere with 21% oxygen, and a magnetic field that protects us from the solar wind. Without that magnetic field, the atmosphere would have escaped into space, as happened on Mars.
It formed about 4.5 billion years ago and since then has gone through periods of extreme volcanism, global glaciations, and mass extinctions. Life appeared at least 3.5 billion years ago and has completely transformed the planet. Photosynthesis changed the atmosphere and made the ozone layer possible.
Let’s be clear: we don’t know if there’s another planet like this one in the universe. The exoplanets we find are light-years away and most are gas giants or hellish worlds. Earth is, for now, unique. And that should make us think twice before destroying it.
Mars: the red desert we’ll visit someday
Mars is the next human target. It has polar caps of water ice and CO2, volcanoes like Olympus Mons (22 km high, the largest in the Solar System), and a canyon, Valles Marineris, that would make the Grand Canyon look like a crack in the sidewalk.
Its atmosphere is very thin, 95% CO2, with a pressure of barely 1% of Earth’s. That means you can’t breathe or survive without a suit. But there’s frozen water beneath the surface and at the poles. NASA’s Perseverance rover has been searching since 2021 for signs of ancient microbial life in Jezero Crater.
The question isn’t whether we’ll go to Mars, but when and who will foot the bill. Crewed missions face radiation, isolation, and a one-way trip of about 7 months. It’s no trivial matter.
Jupiter: the giant that protects us (and sometimes threatens us)
Jupiter is so large that 1,300 Earths could fit inside it. It’s a ball of hydrogen and helium with brutal gravity. Its Great Red Spot is a storm larger than Earth that has been active for centuries.
Its moons are a universe unto themselves. Europa, Ganymede, Io, and Callisto. Beneath Europa’s icy crust is an ocean of liquid water with more volume than all of Earth’s oceans combined. NASA’s Europa Clipper mission, launched in 2024, will arrive there in 2030 to study it.
Jupiter acts as a gravitational shield, deflecting comets and asteroids that could impact Earth. But it can also fling them toward us. It’s not a guardian angel, it’s an unpredictable giant.
Saturn: the rings that disappear (and will return)
Saturn is famous for its rings, but they aren’t eternal. NASA’s Cassini mission discovered that the ring material is falling toward the planet at a rate that will make them disappear in about 100 million years. On a cosmic scale, that’s a blink of an eye.
Its moons are fascinating: Titan has lakes of liquid methane and a dense atmosphere. Enceladus shoots geysers of salty water from its south pole, indicating a subsurface ocean. Both are candidates for hosting microbial life.
NASA’s Dragonfly mission will lift off in 2028 bound for Titan. A drone will fly through its atmosphere to study its chemistry. It’s one of the most ambitious missions of the decade.
Uranus and Neptune: the ice giants almost nobody mentions
Uranus is the planet that rotates on its side, with a tilt of 98 degrees. Probably due to a giant impact in its past. It has an atmosphere of hydrogen, helium, and methane, which gives it that blue-green color. It’s the coldest planet in the Solar System, with temperatures as low as -224 °C.
Neptune is similar, but more massive and with the fastest winds in the Solar System: up to 2,100 km/h. Both have faint rings and icy moons. The only mission that visited them was Voyager 2 in 1986 and 1989. Since then, nothing. There are proposals to send a probe to Uranus in the 2030s, but it hasn’t been approved yet.
Pluto and the dwarf planets: why it’s no longer 9
Pluto was considered the ninth planet from its discovery in 1930 until 2006. That year, the IAU reclassified it as a dwarf planet because it doesn’t meet the third criterion: it hasn’t cleared its orbit of other objects. It shares its region with the Kuiper Belt.
But Pluto isn’t alone. There are other dwarf planets: Eris, Haumea, Makemake, and Ceres (in the asteroid belt). If we had kept Pluto as a planet, we’d have to add many more and the number would be chaotic. The decision was scientific, not arbitrary.
NASA’s New Horizons mission flew by Pluto in 2015 and showed us an icy world with mountains of water ice and plains of frozen nitrogen. Beautiful, but not a planet according to the current definition.
What’s coming: missions that will change what we know
In the coming years we’ll see results from BepiColombo (ESA/JAXA) at Mercury, Europa Clipper at Jupiter, Dragonfly at Titan, and the missions to Venus. There’s also talk of a probe to Uranus and a space telescope to study rocky exoplanets.
Space exploration is no longer just a matter for governments. Private companies like SpaceX, Blue Origin, and Rocket Lab are making access to space cheaper. That means more missions, more data, and more discoveries.
If I’ve learned anything from writing this, it’s that the Solar System isn’t 9 planets or 8. It’s dozens of worlds, moons, asteroids, and comets that still hold secrets from us. And the best is yet to come.
